Enhancement of infrared absorption through a patterned thin film of magnetic field and spin-coating directed self-assembly of gold nanoparticle stabilised ferrofluid emulsion

Molecular vibration signals were amplified by the gold strip gratings as a result of grating resonances and nearby electric field hotspots. Colloidal gold island films exhibit similar enhancement; however, the uneven geometrical characteristics of these films restrict the tunability of the vibrational enhancement. Infrared absorption is enhanced by regular metallic patterns such as arrays of strips fabricated using a top-down approach such as nanolithography, although this technology is expensive and difficult. The significant infrared absorption may serve as tuneable antenna sensitization to improve the sensor performance. In this article, we present a simple one-step process for fabricating optically sensitive ordered arrays of a gold nanoparticle ferrofluid emulsion in polyvinyl alcohol (PVA) using a magnetic field-directed and spin-coating self-assembly (MDSCSA) process. Techniques such as UV-visible absorption, scanning electron microscopy, and grazing-angle infrared spectroscopy were used to evaluate various parameters associated with the nanostructures. Unlike the gold strips, the chain-like features in the iron oxide nanoparticle arrays were discontinuous. The fabricated chain-like ordered arrays have been shown to increase the local field to enhance the infrared absorption corresponding to the symmetric vibration of the –CH2 (2918 cm−1) group present in PVA by ∼667% at a 45° grazing angle, as the chain thickness (CT) increased by 178%. This scalable and simple method can potentially generate low-cost patterns for antenna sensitisation.


Introduction
Infrared (IR) spectroscopy is a rapid, accurate, and robust method for the detection of molecules in a range of applications. For instance, its integration with microuidics has enabled the detection of vascular endothelial growth factor (VEGF) for early disease diagnosis. 1,2 However, the sensitivity of IR spectroscopy is limited when detecting trace amounts of analytes, owing to the small cross-section of its molecular vibrational signals. Surface-enhanced infrared absorption spectroscopy (SEIRA) is commonly used to improve sensitivity. Metal nanostructures, such as gold gratings, have been found to be particularly useful in SEIRA, as the plasmon polariton resonance of gold gratings in the mid-IR region can be matched with molecular vibrational signals to enhance sensitivity. However, fabricating metal nanostructures with precise geometrical features requires time-consuming, complex, and expensive methods such as gold sputtering, electron beam lithography, and nanoimprint lithography. The cost, complexity and scalability may improve by using a simple, rapid, and less complicated magnetically directed self-assembly (MDSA) technique. 3,4 Although MDSA can non-intrusively organize magnetic nanoparticles (NPs) to form arrays of chains 5,6 suitable for applications in several elds such as photonics, 7-9 storage devices, 10 microuidics, 11a,b and optical lters, 12 it is difficult to organize poorly magnetic-responsive (diamagnetic) materials such as gold NPs. Also, the loss of materials from spin coating is a common demerit regarding this fabrication technique. 13 Combining non-magnetic and optically active materials with magnetic iron oxide helps overcome the limitations of MDSA and enables the formation of arrays of non-magnetic colloidal assemblies in the form of grating order that is suitable for optical applications and performance in terms of diffraction and wave dispersion. [14][15][16] This makes this technique advantageous to other homogeneous metal-polymer-composite thin lm layer, however, the irregular and jagged NP chains formed during MDSA processing, coupled with the optically inactive iron oxide NPs, may not produce an optical response similar to that of the nanoimprinted gold strips. Additionally, controlling the lm thickness produced using MDSA is not ideal. To address these challenges, in this work, the MDSA technique was modied to combine with spin-coating to form an array of gold nanoparticle stabilised pickering ferrouid emulsion chains to study the feasibility of low-cost antenna for surface enhanced infrared absorption (SEIRA) spectroscopy. The modied MDSA technique, now termed magnetic directed and spin coating selfassembly (MDSCSA), helped to achieve long-range assemblies of gold NPs in the form of arrays of chains as the non-magnetic gold NPs adsorbed at the interfaces 17b,c of the oil ferrouid oilin-polyvinyl alcohol aqueous solution nano-emulsion droplets, also known as pickering emulsions aligned to external magnetic eld. 18 It is hypothesized that the clusters of gold and iron oxide NPs would contribute to the electron dynamics to enhance the local electric eld, leading to an increase in the infrared-induced molecular vibrational signal [19][20][21] and contribute to random light scattering 22 or interference. 23,24 The interfacial interaction between gold and iron oxide causes the diffusion of the excited electrons from the Fermi level of gold NPs to the conduction band of the iron oxide, causing charge accumulation at the defect sites of the interface, therefore improving the optical activity of iron oxide. 25,26 It is expected that the iron oxide in the ferrouid droplet will interact with the adsorbed gold NPs and localize the electrons to increase the infrared molecular vibrational signal. The advantage of iron oxide as an electron reservoir creates an opportunity for minimizing the required gold concentration in the entire composite system. 26,27 Therefore, it is benecial to maintain the required concentration of gold NPs in the chains, the appropriate length and thickness of the chains, and gaps between chains. During magnetic eld directed self-assembly, the dipole-dipole interaction between magnetic particles or magnetic uid droplets builds arrays of chain-like clusters that can span over a long distance at the microscale along the generated ux lines of the magnetic source. 28,29 The density of these ux lines is dependent on both the size and strength of the magnetic eld; 30 therefore, the chain length, thickness, and gaps between them can be controlled by suitably selecting the magnetic eld strength, orientation, and distance from the subject. Additionally, these arrays of chains must be immobilized on the substrate, either by drying or curing an aqueous polymer. 31,32 The composition considered in this study was an oil-based ferrouid (hydrophobic phase) dispersed in polyvinyl alcohol (hydrophilic phase). This system tends to maintain the sphericity of the droplets and provides exibility for the containing iron NPs to align and drive droplets to form chain arrays with respect to the magnetic eld direction, as well as to maintain electrostatic, stearic, and viscous hindrance between droplets, preventing them from coalescing. 33,34 Polyvinyl alcohol (PVA) system can act as an absorber for analyte molecules in such composite systems for sensory applications, like in the case of silver-PVA composite system for the detection of amines. 35 The applied magnetic eld can control the chain arrays and degree of orderliness; however, the introduction of spin coating contributes to the formation of thin-lm mono-layered chain arrays. Spin coatings can also contribute to the rapid drying of most hydrophilic polymers. [36][37][38][39] In this process, the contention between the magnetic force pulling the droplets towards the magnetic source and the inertial centrifugal hydrodynamic force of the uid from the substrate spinning affects the distribution of the droplets and their chain clusters. Therefore, the process was optimized to control the chain morphology. This method allows the formation of a precise nanostructure that controls the plasmon excitation modes and shis in infrared absorption at different incidence angles of light. This outcome is useful for the development of sensitive biomedical sensors. [40][41][42] Comparatively with MDSCSA, other patterned thin lm nanofabrication techniques require costly equipment and, in most cases, require clean room to build lithographic template for guiding nanoparticles into arrays. 43,44 Also, the application of spin coating in MDSCSA is one of the quickest methods for creating thin lms, thus making product throughput high. 43,45 Amongst other established fabrication techniques, nanoimprinting for example, is one of the fastest and most pristine methods for building patterns. 46,47 However, MDSCSA potentially offers rapid exibility by switching magnetic congurations and orientations to form different patterns, while nanoimprint would require redesign of template using costly lithographic equipment. 48

Methods
This section is divided into two main sub-sections. The rst subsection explains the preparation of gold nanoparticles stabilized pickering ferrouid emulsion. However, this sub-section involves many steps such as the preparation of oleic coated iron oxide nanoparticles, ferrouid, ferrouid emulsion, polyethylene glycol 40S coated gold nanoparticles (PEG-C-GM) through colloidal route and nally, PEG-C-GM nanoparticle stabilized pickering ferrouid emulsion (PEG-C-GM-pi-FF) in a PVA aqueous solution. In the second sub-section, PEG-C-GMpi-FF emulsion dispersed in PVA aqueous solution was used to prepare the thin lm using MDSCSA method. The thin lm contained the array of parallel chains of PEG-C-GM-pi-FF emulsion droplets.

Gold nanoparticle stabilised pickering ferrouid emulsion preparation
As seen in Fig. 1, oil-based ferrouid [49][50][51] and gold methacrylate colloid were prepared (ESI S1.18.1 and S.1.18.2 †). Subsequently, the PEG-C-GM nanoparticles were separated and dispersed in DI water. All materials and equipment are listed in ESI S1.17 and S.1.17.1. † The prepared aqueous suspension of PEG-C-GM (4 mL) was dispersed in 20 mL of DI water in a sonicator bath for 5 min. Aerwards, 70 mL of oil based ferrouid was added, and the resulting mixture was stirred at 800 rpm using a rotor-stator for 8 min to create micron-sized emulsion droplets (∼400 mm diameter) of ferrouid. Furthermore, it was irradiated for 10 minutes with trains of ultrasound pulses with a central frequency of 20 kHz using a probe-type sonicator, MSE® Soniprep 150, to reduce the diameter of the ferrouid emulsion droplets. Each pulse duration was maintained at 10 seconds, while the transducer displacement was 10 mm. The system was kept cool (∼19 ± 2°C) by inserting it into a thermo-regulating jacket. Six more batches were prepared using the same procedure. To reduce the degree of polydispersity, 15 millilitres of emulsion were centrifuged at a precisely determined speed of 1000 revolutions per minute for 10 minutes. Prior to determining the optimal speed and time of the size control process, a series of tests were conducted to optimize the size distribution. This nanoemulsion was termed polyethylene glycol-coated gold methacrylate pickering ferrouid (PEG-C-GM-pi-FF). This process is shown in the third row of Fig. 1.
Separately, the 5 wt%, 3.3 wt%, 1.7 wt% and 1.3 wt% of aqueous PVA solutions were prepared by adding PVA powder into DI water and stirring at 200 rpm for 10 minutes. The densities and viscosities of the resulting solutions with different concentrations are presented in ESI Table S2. † Subsequently, the solutions were heated at approximately 80°C for 2.5 hours until they turned transparent. Five millilitres of the pickering emulsion were then added to 10 mL of an aqueous solution of PVA and stirred at 400 rpm for 1 hour using a rotor mixer.

Thin lm preparation using MDSCSA method
As shown in Fig. 2a-c, the experimental setup was designed to prepare the thin lm containing arrays of chains of gold nanoparticle-stabilized pickering ferrouid droplets. To prepare the thin lm, a substrate was xed on the base with the help of a support. The magnet was placed approximately 5 mm below the surface of the substrate as shown in Fig. 2d, and it was at the centre of the substrate as well (a top-view in Fig. 2b). A strong cubic neodymium magnet having 2.5 mm length and magnetic eld strength of 202 mT on surface was placed beneath the surface of the substrate measuring 76.2 mm × 25.4 mm (Fig. 2b). Fig. 2c shows the magnetic eld distribution in mT across the substrate.
The setup was spun using an SCS™ 6800 spin coater at a range of speeds programmed to operate within a specied timeframe. The spin coater was set to spin at the maximum speed for 50 seconds aer accelerating for 5 seconds. Finally, the spinner was decelerated for 5 seconds to bring it to a standstill, making it a total spinning time of 60 seconds. The glass slide substrates were cleaned to remove debris on the surface with 70 wt% aqueous isopropanol solution and then dried in an oven at 60°C for one hour. The effects of this treatment were tested using contact-angle measurements. Prior to starting the spinning process, 0.5 mL of PEG-C-GM-pi-FF emulsion dispersed in PVA aqueous solution was allowed to settle on the substrate for 60 seconds, providing sufficient time for the droplets to interact with the magnetic eld and build chain clusters. Aer 60 seconds, the substrate was spun in spin coater at various speed and time. Different viscosity of PVA aqueous solution was also used for optimising the thin lm patterns. The thin-lm coating was found to be dried only when it was spun at speeds above 700 rpm for 60 seconds, allowing the formed chains to immobilize on the substrate. Aer optimising the speed, viscosity and spinning time combinations, the array of chains was prepared by spinning the substrate laden with PEG-C-GM-pi-FF having a PVA viscosity of 15.2 mPa as made from 3.3 wt%.

Results and discussion
During the synthetic preparation of the PEG-C-GM-pi-FF emulsion, several characterization methods were conducted. These methods included the measurement of droplet size, zeta potential, UV-visible absorption, thermal gravimetric analysis, and contact angle. Additionally, intermediate products such as PEG-C-GM, oleic acid-coated magnetite nanoparticles, and fer-rouid were also characterized for relevant properties, including magnetic properties. Furthermore, the morphology of the prepared thin lm was discussed. Lastly, Specular Fourier Transformed Infrared Spectroscopy (SR-FTIR) was performed on the patterned thin lm to show enhanced absorption (surface enhanced infrared absorption-SIERA) and the correlation of absorption peaks with the lm's morphology.

Size measurements of nanoparticles and emulsion.
In the characterization of emulsions, the sizes of oleic acidcoated magnetite (OCM), gold methacrylate (GM), PEG 40S coated gold methacrylate nanoparticles, and PEG-C-GM-pi-FF emulsion droplets were estimated using TEM images. The fer-rouid was dispersed and allowed to dry on a TEM copper grid (300 mesh). The oleic-coated magnetite (OCM) NPs, with an average diameter of 15 ± 3 nm as shown in Fig. 3a, were captured on the grid. The oleic acid coating helped to maintain the stability of the NPs and prevent occulation.
The average hydrodynamic diameter of gold methacrylate (GM) and PEG-C-GM NPs was determined separately using dynamic light scattering (DLS) with the Malvern zeta sizer. The hydrodynamic diameters were found to be 18.6 ± 3.9 nm and 28 ± 7.4 nm, respectively (ESI Fig. S2 †). However, the average mean diameters estimated using TEM were 17 ± 2.6 nm for GM NPs (Fig. 3b) and 22 ± 4 nm for PEG-C-GM NPs (Fig. 3c).
The zwitterionic PEG 40S molecules on the GM NPs facilitated their adsorption on the ferrouid droplets dispersed in the PVA aqueous solution, as observed in the TEM image (Fig. 3d). In the TEM image, the PEG-C-GM NPs appeared as small dark particles on a large droplet of the ferrouid. The average diameter of the PEG-C-GM-pi-FF emulsion, measured using the Malvern zeta sizer, exhibited a bimodal ferrouid droplet size distribution. The distribution had two peaks with central mean diameters of 610 ± 240 nm and 170 ± 16 nm, respectively (ESI Fig. S4 †).
To narrow the entire size distribution, centrifugal action was applied to the PEG-C-GM-pi-FF droplets, segregating larger droplets from smaller ones within a specic time frame. A total of 70 000 revolutions were required to achieve a size distribution of approximately 220 ± 50 nm (see ESI, Fig. S5 †).
3.1.2 Zeta potential. Zeta potential of deionised water (DI), PVA in DI, PEG-C-GM in aqueous PVA solution and PEG-C-GMpi-FF in PVA solution were measured and presented. The attachment of methacrylic acid molecules led to an increased negative charge on the surface of gold nanoparticles, with a value of −29.8 mV (ESI Fig. S3 †), at a pH of 5.6, which is lower than that of gold-acrylate. 52 However, the zwitterionic PEG 40S coating reduced the charge on the gold NPs by acting as a graing layer. 53 However, the zwitterionic PEG 40S coating reduced the charge on gold NPs by acting as a graing layer. 53 The PEG-C-GM nanoparticle-stabilized pickering ferrouid emulsion exhibited a lower electronegativity due to the low zeta potential of the ferrouid in the PVA aqueous solution. Based on observation that the ferrouid emulsion was solely stabilized through the adsorption of gold nanoparticles, it can be assumed that the overall charge depended on the number of NPs attached to the surface of the ferrouid droplets. 54 Furthermore, the negative zeta potential of PEG-C-GM-pi-FF in the low-zeta potential PVA aqueous solution conrmed the adsorption of gold nanoparticles on the oil droplet. Additionally, the oil droplets were large enough to reduce the overall surface area and zeta potential. The PEG coating, acting as a layer of water-swollen gel, generated supplementary steric repulsions between gold NPs. 53 Aqueous PVA exhibits low electronegativity, which makes it suitable for working with pickering emulsion. In emulsion, two electronegative charges repel each other, eliminating the possibility of chemical bond formation that could potentially distort the creation of droplet dipole-dipole chains when an external magnetic eld is introduced.
ESI Fig. S3 † displays the zeta potentials of PEG-capped and uncapped gold methacrylate NPs with the zwitterionic characteristic of PEG.
3.1.3 UV-visible absorption spectroscopy of emulsions. The UV-visible absorption spectrum of 5 mL of ferrouid redispersed in 20 mL of cis-cyclooctene was obtained. The resulting spectrum in Fig. 3e matches that obtained in a previous report. 55 UVvisible absorption spectra were acquired for PEG-C-GM (Fig. 3e). The surface plasmon vibration of Au colloids appeared at 528 nm wavelength, which is characteristic of spherical Au NPs. 52 As seen in the Fig. 3b, the prepared GM colloids showed a typical red colour of gold. 52,56 The methacrylate ion adsorbed on the gold nanoparticles maintains the charge around the particles through hydrophobic-tail interactions, as shown in the schematic in Fig. 3b and prevents occulation. The UVvisible absorption spectrum of the PEG-C-GM-pi-FF emulsion (Fig. 3e) consisted of a broad absorption band spanning from range 515 nm to 610 nm. It is similar in terms of attaching gold nanoparticles to an iron oxide particle core, as presented in previous works. 57, 58 3.1.4 Thermal gravimetric analysis and contact angle. Thermal Gravimetric Analysis (TGA) revealed that the thermal degradation of methacrylate on the GM surface initiated at approximately 270°C and completed at 420°C, resulting in a 6.5% loss of mass (see Fig. 3f). For PEG-C-GM, there was an initial gentle decline in mass observed between 30°C and 260°C , followed by a steep decline as the temperature rose to 420°C, showing a total mass reduction of approximately 12.8%. The number of methacrylate molecules per gold nanoparticle surface was calculated to be 25 665 molecules (see ESI S1.12 † for the calculation step). 59 Images of the droplets for different PVA concentrations were analysed using the contact angle plugin of ImageJ® soware. 60 The contact angle between the glass slide and the PVA droplet increased with increasing PVA concentration (ESI Fig. S6c †). A typical image in ESI Fig. S6a and b † shows a contact angle of 16.9°for a 3.3 wt% solution, conrming the good adhesion of the PVA solution to the substrate. The interfacial tension between PEG-C-GM-pi-FF and the glass slide in DI water, based on the (LBADSA) plugin of ImageJ®, 61 gave a contact angle of 174.88°. The image scale was set to 316 pixels per mm (Fig. 3g).
3.1.5 Magnetic hysteresis. A superconducting quantum interference device (SQUID) was used to measure the magnetic moments. As seen in ESI Fig. S1A, † the saturation magnetization (M d ) for bulk Fe 3 O 4 is 446 kA m −1 , and the saturation magnetization (M s ) for magnetite (Fe 3 O 4 ) and OCM nanoparticles are 374.8 kA m −1 and 219.9 kA m −1 , respectively. The magnetic susceptibility (X iL ), which is the initial slope of the curve intercepting at zero magnetic eld for Fe 3 O 4 and OCM, is 8.6 and 1.5, respectively. The coercivity of Fe 3 O 4 nanoparticles and OCM from ESI Fig. S1B and C † are 1.8 kA m −1 and 1.3 kA m −1 , respectively. The obtained experimental data were used to determine the effective diameter and standard deviation of the tested materials using the model 62,63 (see ESI S1.8 and Table S1 †).
The effective diameter of 13.7 ± 2.94 nm estimated using this method was similar in terms of diameter and standard deviation to the one obtained from the TEM micrograph of OCM in Fig. 3a. The difference between the estimated diameters obtained using the two methods could be attributed to the coating layer (oleic acid), which reduced the effective magnetization of magnetite (Fe 3 O 4 ). On the other hand, the estimated diameter (14 ± 3 nm) of the magnetite NPs was much closer to the average diameter obtained by TEM micrograph. The saturation magnetization for the ferrouid (OCM dispersed in ciscyclooctene) obtained was 29.5 kA m −1 . This yielded a magnetization ratio between the OCM particles and ferrouid of 0.06.

Patterned thin lm characteristics
3.2.1 Morphology of thin lm. MSCDS processing of PEG-C-GM-pi-FF in PVA yielded a thin lm on a glass slide, as shown in Fig. 4a. Optical and scanning electron microscopy of the thin lm showed arrays of parallel chains of PEG-C-GM-pi-FF on the substrate with varying spatial densities. Therefore, the image shown in Fig. 4a was divided into three regions. A typical dark-eld image of the thin lm (region 2) coated with PEG-C-GM-pi-FF in PVA is shown in Fig. 4b.
In all cases, drying started from the outer edges of the substrate and was directed inward, generating a circular pattern whose diameter decreased with increasing spinning time. The lm in the region of interest was divided into three regions, as shown in Fig. 4a. Fig. 4g-i show the corresponding chain arrays at higher resolution (1820 pixels per mm), where emulsion droplets are visible with white rings appearing as corona of the adsorbed gold nanoparticles at the circumference. Due to the high magnetic strength at the centre of the magnet, thicker columns with worm-like or labyrinth-like formats were established. The chain length (CL) of PEG-C-GM-pi-FF was found to be shorter and densely packed at the centre (Fig. 4d) of the lm, but it increased in length at a location away from the centre (Fig. 4e). However, chain formation was no longer visible at a location far away from the centre of the rotation of the lm according to the SEM image (Fig. 4f), and that location was not included in the image analysis. These chains were separated by a small or negligible gap. To formally represent the geometrical features of these chains on the substrate, the chain gap (CG), chain length (CL), and chain thickness (CT) are dened in Fig. 4c. Some thin-lm nanostructures possess two-fold symmetries indicating a long-range ordering of particles. 64 CL, CT, and CG were quantied using image analysis (ESI Fig. S7 †) to estimate the effect of the geometrical features of the patterned thin lm. Further details related to the image analysis are provided in ESI S-1.14 and S-1.15. †

Specular reectance Fourier transformed infrared spectroscopy (SR-FTIR)
The infrared vibration signals from the PVA adsorbed on the PEG-C-GM-pi-FF arrays in thin lm (prepared using 15.2 mPa as viscosity solution at a spin speed of 2500 rpm) were observed. Please refer to ESI S1.7 † for the description of the SR-FTIR method.
The complete FTIR spectrum at angles of incidence (20°, 45°a nd 82°) can be found in ESI Fig. S8, † covering the range of 4000 cm −1 to 650 cm −1 . To process the different spectra, baseline corrections were performed using Origin® soware. For the selected range of 3200 cm −1 to 2600 cm −1 , the reectance baseline was established at 2600 cm −1 for 20° (Fig. 5a) and 45° (  Fig. 5b). The baseline for 82°spectrum can be seen in ESI Fig. S9. † The range from 1900 cm −1 to 1600 cm −1 did not require a baseline reference as the vibrational intensities were sufficiently signicant for differential analysis. This can be observed in 20° (Fig. 5c) and 45°(Fig. 5d) reection spectra, while the reection spectrum for 82°can be found in ESI Fig. S9. † 3.3.1 Peaks in range -3200 cm −1 to 2600 cm −1 . The broad absorption band observed at 3600-3000 cm −1 (ESI Fig. S8 †) corresponds to the OH stretching vibrations in PVA molecules.  Additionally, the vibration between 2923-2900 cm −1 can be attributed to the asymmetric stretching of the -CH 2 group in the PVA molecules within the prepared arrays of PEG-C-GM-pi-FF chains on the glass substrate. 65,66 In the Fig. 5a and b, it can be observed that the intensity of the symmetric stretching vibrational mode at 2846 cm −1 associated with the -CH 2 , 67 increased relative to the asymmetric vibrational intensity (∼2918 cm −1 ) and in relation to the radial distance for both grazing angles. The vibrational intensity of both -CH 2 peaks was highest in the spectra obtained from the centre of the lm (r = 0 mm) and decreased as the radial distance from the centre (r = 7.2 mm) for both grazing angles. Thicker chains with a higher concentration of gold NPs are likely to contribute to the increase in vibrational peak intensity. This observation is consistent with previous ndings that demonstrate an increase in vibrational intensity of both peaks (2918 cm −1 and 2845 cm −1 ) in relation to the concentration of gold NPs within the PVA gold composite. 68 The interaction between gold and iron oxide NPs in the arrays of PEG-C-GM-pi-FF chains potentially enhances charge transfer between all NPs to propagate the electromagnetic eld throughout the PEG-C-GM-pi-FF chains in the thin lm. 69 3.3.2 Peaks in range -1900 cm −1 to 1600 cm −1 . In Fig. 5c and d, the FTIR spectrum of the PVA adsorbed on the PEG-C-GMpi-FF chain arrays revealed the presence of vibrational modes at 1367 cm −1 and 1409 cm −1. These modes are associated with -OH bending in C-H wagging or -CH 3 stretching and C-H deformation, respectively. The vibrational mode observed at 1707 cm −1 can be attributed C]O vibrations, 66,67 while the ∼1729 cm −1 was ascribed to C]O vibration associated with acetate molecules within the PVA (88% hydrolysed). 70 At the centre of the substrate (r = 0), there was a relative increase in the intensity of the C]O (1707 cm −1 ) vibrational mode compared to the C]O (1729 cm −1 ) vibrational mode. However, the intensity of both peaks decreased in all spectra obtained from locations on the thin lm at a high radial distance (r > 7 mm from the centre of the lm). The difference in vibration signals was related to the chain thickness-chain gap ratio (CT/CG) and chain thickness (CT) for angles of incidence of 20°and 45°. The spectra for the 82°angle are presented in ESI S1.16. † 3.3.3 Surface morphology dependent surface enhanced infrared absorption. The nano-printed gold grating enhances the vibrational modes of the monolayer of the chemical analyte. Leading to local eld enhancement, as demonstrated by the Specular Reectance Fourier Transform Infrared (SR-FTIR) technique. 71 The vibrational band frequencies of the adsorbed molecules scale with the size and period of the grating, 72 resulting in the amplication of the local electric eld. To investigate the effect of chain thickness and gaps in the fabricated thin lm on the infrared (IR) absorption of adsorbed PVA molecules, the peaks of SR-FTIR spectra obtained at angles of 45°, 20°, and 82°were correlated with the CT/CG ratio. The same locations on the thin lms containing chains of PEG-C-GM-pi-FF adsorbed with PVA were used to acquire the SR-FTIR spectra and images for deriving the CT/CG ratio. In Fig. 5a-d, the vibrational mode intensities of the local minima points (yellow circles) were subtracted from the maximum vibrational mode intensities (red circles) associated with each spectral position to determine the surface-enhanced absorption signals. Subsequently, the obtained vibrational signals were correlated with each CT/CG ratio ( Fig. 5e and f). It should be noted that the corresponding values of r (mm), CT (mm), and CT/CG were correlated (see ESI Table S5 †); therefore, these terms are used interchangeably in this section.
The relationship between CT/CG and the vibrational signals of PVA-CH 2 -symmetric and PVA-CH 2asymmetric was found to be nonlinear at all angles of incidence (20°, 45°). The -CH 2 asymmetric and symmetric vibration resonance modes ascribed to the methylene group present in PVA were of the same intensity as those at r = 7 mm ( Fig. 5a and b). At 20°incidence, the vibration signals of the -CH 2 -asymmetric band increased with the CT/CG ratio (Fig. 5e). A gentle increase in vibrational intensity up to the highest CT/CG ratio at 2845 cm −1 was observed. The -CH 2 symmetric and asymmetric vibrational signals increased by 350% and 39%, respectively, with an increase in the CT/CG ratio of 170%, and a similar increasing trend for chain thickness (ESI Table S5 †) was also observed. The charge interaction between iron oxide-gold NPs would have increased with an increase in the CT/CG values. The increased charge transfer at high CT/CG values may have increased the vibrational signals. The increase in the PVA-CH 2 -symmetric and asymmetric vibrational signals was more pronounced at 45°incidence angle by 667% and 94%, respectively (Fig. 5f), because of the increase in the optical beam area and path within the patterned thin lm. The increase in the vibrational signal intensity was 58% at 20°angle (ESI Fig. S9a †).
The reduction in the gaps between PEG-C-GM-pi-FF chains combined with increased chain thickness, also provided a dense hot spot volume and surface sites for the interface between PEG-C-GM-pi-FF droplets, leading to a broadband local eld enhancement. 67 This broad band eld enhancement could play a role in enhancing the absorption of the spectrally distributed vibrational bands. 71 The gold-iron oxide NPs interface provides an enhanced local electric eld when in proximity with another, especially where the cluster is denser with little gaps between particles, as seen in some gold assemblies. 69 Band CA-CO and PVA-CO also responded to the trend of chain thickness for all incident angles, where they had approximately the same vibration signal intensities at both 20°and 45° (Fig. 5e and f). At 20°i ncident angle, the change in CA-CO and PVA-CO vibrational signals in the spectrum obtained from area on thin lm, (where CT values were between 1.01 mm to 2.82 mm) was 16% and 17% respectively. At 45°incident angle, this decreased by 39% for PVA-CO-CO peak and increased by 19% for CA-CO peak under the same range. Meanwhile, at 20°, this decreased by 22% for PVA-CO and decreased by 16% for CA-CO within the same range. This was because of the absorption value at these bands for a single-layer PVA lm, which was the same as the value at r = 0 mm (Fig. S9b †). However, there was a noticeable decrease in the intensity of the peaks at lower CT/CG values (Fig.S9c †).
3.3.4 Grazing angle optimisation. The increase in the grazing angle improves the interaction of light with the thin lm owing to spatial extension but decreases the optical density. While increased spatial extension enhances the vibration signals, decreased optical density reduces the detection sensitivity. The vibrational signal intensities were highest in the spectra obtained at 45°grazing angle (Fig. 5f) and lowest at 82°grazing (Fig. S9c †). At 45°, the optical density and spatial coverage of the PEG-C-GMpi-FF chains were optimized to obtain the highest vibrational signals. These angles may be particular to the geometry of the thin lm generated in this study because the greater grazing angle on the chains with a given height might cast a shadow of one strip on to the other, limiting the overall exposure of light on the material. 70 Another possibility is that incident wave backscattering increases at greater grazing angles. 73 Therefore, the optical waves interacted poorly with the thin-lm pattern and glanced at the highest grazing angle. Overall, the magnetically directed self-assembly established near and far-eld interactions through good dispersion, providing tunability. [73][74][75]

Conclusion
This study demonstrates the utilization of magnetic directed spincoating self-assembly (MDSCSA) to fabricate an optically sensitive lm consisting of periodic nanoarrays of gold nanoparticlestabilized pickering ferrouid emulsion chains in PVA on a silica glass substrate. The variation in the chain thickness (CT) and gaps (CG) between the chains enhances the vibrational signal of the CH 2 infrared absorption bands. The enhancement in the CT/CG ratio from 0.11 to 2.87 resulted in a 667% increase in the infrared vibration signal of the CH 2 (2918 cm −1 ) bond (in comparison to 2845 cm −1 ) at 45°beam incidence, while the vibrational signals dropped at 20°angle. These results suggest the potential of using gold nanoparticles with Fe 3 O 4 to create tuneable infrared resonant peak strips for surface-enhanced infrared spectroscopy. The structure of this lm combines both far-and near-eld effects to locally enhance the charge and vibration of the attached molecules. The variability in the resolution of a single thin lm allows for greater exibility in identifying and comprehending species traits under different CT/CG patterns. Future work will involve investigating the minimum quantity of gold nanoparticles required in relation to Fe 3 O 4 nanoparticles to establish both near-UV and near-infrared plasmon effects.

Data availability
A open access link to the raw data will be provided aer the acceptance of the manuscript.

Conflicts of interest
There are no conicts to declare.